High-pressure metal halide discharge lamps are increasingly used for general illumination. It has been proposed to use such lamps also as headlamps of vehicles, and particularly automotive vehicles. Headlamps require only comparatively low power light sources, that is, less than 100W and especially less than 50W; a power rating of 35W has been proposed. The quality of the obtained light, that is, its color index and radiation temperature is suitable for use in headlamps. Known fills can satisfy the requirements. For rapid starting, preheating of the electrodes is desirable.
British Pat. No. 530,376 describes a high-pressure discharge lamp with two melt connections. A heater winding is located about a portion of the discharge vessel which is located in a capillary opening of quartz glass, coupled to the discharge vessel. The heating filment is operable separately from the lamp. The fill substances within the discharge vessel are intended to be vaporized by the separate heating winding, so that the starting time of the lamp is reduced and full light output is more rapidly obtained than without preheating.
U.S. Pat. No. 2,820,164 decribes a high-pressure discharge lamp which is heated at both ends. The lamp is designed for combination with an optical system. A plurality of ceramic rods, each supplied with a heater resistance wire, are located about the ends of the discharge vessel, spaced therefrom, in order to provide radiant heat towards the discharge vessel to heat the discharge vessel and to maintain the fill substances therein in vaporized condition. Instantaneous light output from the lamp can be obtained thereby. The mechanical structure of such a heater arrangement is complex and heat transfer to the discharge vessel from the outside thereof is not efficient; the arrangement is not suitable for use with very small lamps, for example intended for vehicular applications.